Field of the Invention
The present invention relates to an image capturing apparatus and a method for controlling the image capturing apparatus that performs autofocus control.
Description of the Related Art
As an autofocus (AF) method of an image capturing apparatus, a contrast AF method and a phase-difference AF method are known. Both the contrast AF method and the phase-difference AF method are AF methods that are often used in video cameras and digital still cameras, and in some of these AF methods, an image sensor is used as a focus detection sensor. In these AF methods, focus detection is performed using an optical image, and accordingly there are cases where an aberration of an optical system that forms the optical image causes an error in a focus detection result. A method for reducing this kind of error has been proposed.
Meanwhile, it is known that an aberration of an optical system of a master lens when a converter lens is attached is magnified by a magnifying power of the converter lens.
Japanese Patent No. 3345890 discloses a method for converting a correction amount for focus detection to an amount corresponding to a square of image sensing magnification of a converter lens, and further adding a correction amount for focus detection corresponding to an aberration of the optical system of the converter lens to the converted value, thereby correcting the focus detection result.
However, with the method in Japanese Patent No. 3345890 that magnifies the focus detection error of the master lens by the square of the magnifying power of the converter lens, a problem arises in that a focus detection error cannot be sufficiently corrected. This is because not only the focus detection error of the converter lens is converted by the square of the magnifying power in the vertical direction, but also the magnifying power in the horizontal direction causes changes in characteristics due to the focus detection area and frequency characteristics in image shooting.
Furthermore, the focus detection error is originally a difference between an aberration state in which an observer feels that a captured image is well focused and an aberration state that the focus detection result shows.